1. FIELD OF THE INVENTION
This invention relates to a toner composition for use in electrophotography. More specifically, this invention relates to a toner composition for use in electrophotography that can be uniformly, strongly and negatively charged in either the direct or indirect electrophotographic developing method, thereby developing or rendering visible a positively charged electrostatic latent image on a copying substrate, to give a visible image free of fogging.
As is well known, xerography is an electrophotographic method utilizing static electricity. There have been practiced several methods for developing the electrostatic latent image which is formed on the surface of an insulating substance. Commercially and widely employed methods includes the cascade method in which glass beads are used as a carrier for the toner and the magnetic brushing method in which iron powder is used as a carrier. In both of these methods the toner particles are electrified by friction and adhered onto the electrostatic latent image by means of electrostatic attraction.
A toner is a colored resin powder which, when charged by friction, either by the cascade method or the magnetic brushing method, should be charged uniformly either entirely positive or entirely negative, in order to make it possible to obtain a developed visible image free of fogging.
When the non-exposed portion of the surface of the photosensitive layer forms a positive electrostatic latent image after having been charged and exposed, a positive developed image will not be obtained during development unless developing is effected using a negatively charged toner. It is known that when a toner composed of a known polystyrene or polyester resin and which contains non charge-control agent is charged by friction by the cascade method or the magnetic brushing method, it is electrified with only a weak positive or a weak negative charge. It is difficult to electrify such toners with a negative charge that is strong enough to produce a developed image having a high image density and clarity. When it is desired to electrify such a toner with a strong negative charge, it is known to add an azo type oil-soluble dye containing a complex compound of chromium, as a charge control agent, as reported in Japanese Pat. Publication No. 26478/70. But a major disadvantage in controlling the electrification of the toner by friction by adding a charge-control agent is that the charge-control agent usually is difficult to blend uniformly in the thermoplastic resin, so that a non-uniform dispersion is formed which makes it difficult to obtain a uniform negative charge on the surface of the toner grains, thus resulting in fogging or trailing during developing and eventually making it hard to obtain a clear developed image.
To overcome such disadvantages, there should be used a thermoplastic resin that by its inherent nature is substantially and strongly electrified negative, by friction, without the aid of a charge-control agent. According to the Study of Friction Electrification of Resins (for example, "Electrophotography" by Isao Shinohara, Vol. 10 No. 1, pp. 2-13, 1971), it is known that Saran. Teflon and polyvinyl chloride have a greater tendency to be electrified negative, in comparison to polystyrenes. These halogenated thermoplastic resins have a potential of use as a toner resin from the viewpoint of their electrification properties, but they lack other physical properties required for toner resins. Such halogenated thermoplastic resins fail to meet the following requirements for toner resins:
1. melt sharply at relatively low temperatures; PA1 2. do not develop blocking or caking at ordinary (20.degree. to 25.degree. C) temperature; PA1 3. easy to pulverize; PA1 4. good stability; and PA1 5. good blending with a colorant. PA1 1. a dicarboxylic acid component containing 50 mole percent or more of a dicarboxylic acid, or an anhydride thereof, selected from fumaric acid, maleic acid, succinic acid, maleic anhydride and succinic anhydride, with PA1 2. a polyol component containing more than 50 mole percent of a glycol having the formula ##SPC3##
Owing to the foregoing reasons, halogenated thermoplastic resins are not used in practice. Furthermore, these halogenated thermoplastic resins, at the time of heat fixing of the toner, undergo thermal decomposition evolving halogen vapor and causing discoloration of the colorant as well as corrosion of the metallic parts of the machinery.